The growth of plants in containers has been practiced since before recorded history. During the development of the art of plant growth and the sciences of horticulture, forestry, agriculture, etc., countless improvements in the culture of plants have been made. A portion of these improvements have related to improved environments for root growth and development. For example, it is now common practice to use growing media which have better balances between air filled pores and water filled pores than normally occur in a container filled with naturally occurring mineral, synthetic and/or organic media. A commonly used media that provides a good balance between media, air and water is one prepared from mixtures of aggregates such as perlite or vermiculite and peat moss, often with significant amounts of normal soil being added. Generally, these new media result in improved plant growth as evidenced by faster growth rates, improved yields, and higher quality plants.
However, while it is well recognized by those people skilled in the art that soil media used in containers must have very special properties, the mass of soil present in horticultural containers is often relatively small and shallow, and these two points create problems peculiar to this system. The smallness of the media mass means that the amount of water held is relatively small but shallowness creates saturation (over-water) problems commonly referred to as the "perched water tables". This latter point will be discussed in detail below.
It is well known that plant roots require both oxygen and water to grow properly. In addition, the carbon dioxide and other gases produced by the plant roots and/or microbial action should be permitted to escape from the root environment since they may have a detrimental effect on root and hence total plant development. The importance of root aeration may be simply demonstrated by comparing the root growth in the ground, a clay pot, and a plastic pot. Plants growing in good topsoil in the ground will have a reasonably homogeneous distribution of roots throughout the area referred to as the root ball. Plants growing in a clay or other porous container with the same topsoil will have their roots distributed throughout the soil mass but with a somewhat greater incidence of roots occuring near the outside edges and bottom of the soil mass. Plastic pots, containing the same topsoil, because of their non-porous nature, provide a different environment for the plant root and this often results in a very large percentage of the roots near the bottom where the drainage holes provide a means for aeration.
Interpretation of the heretofore mentioned three examples is as follows: Good soil in its native habitat normally exhibits good capillarity and structure to allow for sufficient water and air (oxygen) movement (aeration) to supply plant roots. Plants grown in porous (clay-type) containers have a preponderence of roots near all surfaces of the container, namely, where there is better gas exchange. Plants grown in non-porous containers (plastic type) normally have their roots better developed in the proximity of the drainage holes, where good aeration exists. Thus in all three cases, allowing better aeration results in more root growth which in turn allows for better total plant growth and development.
However, the economics of pot manufacture suggests that if a means could be found to properly aerate a plastic pot, then such plastic pots would significantly reduce costs to nurseries. Ultimately the public would benefit from the lowered prices. Accordingly, this invention is directed toward better aeration for pots composed of nonporous materials although, the inventive concept would be useful for clay or other such porous type potting materials.
L. H. Slotzy et. al. (Hilgardia. Vol. 35, No. 20, Oct. 1964) reviewed the literature and demonstrated a relationship between O.sub.2 content and its rate of replenishment in the soil and plant growth. Their work, and that of many others, can be summarized thusly: Plant roots have a critical need for oxygen and the media in which they are growing must permit a certain rate of oxygen diffusion to insure adequate or optimum growth. It follows from this fact and a knowledge of the diffusional characteristics of gases that to obtain proper oxygen diffusion, there must be an adequate number of open pores in the soil media, located a reasonable distance from the soil-air interface, and not blocked by sections containing water. It is necessary to have the diffusion of O.sub.2 occur predominately through open pore spaces since the rate of O.sub.2 diffusion through water is 10.sup.4 to 10.sup.5 times slower than through air. Thus, reasonably thin films or layers of water will impede the movement of oxygen sufficiently to produce less than optimum growth.
The relationship between open-pore or air filled pore space and water-filled pore space in a growing media in containers depends upon a number of factors including the composition of the media, its degree of compaction, and the physical shape of the container. This invention relates to a modified container structure to enhance air and water movement and, hence, to improve drainage and/or aeration.
Spomer has pointed out (HortScience, Vol. 9 (2) April, 1974) that container grown plants can often be repeatedly subjected to too much water or too little water. These phenomena arise because the lower portion of the media in an irrigated but drained container is often saturated with water. Furthermore, the water content decreases with height with a corresponding increase in open pores or air space. The physical basis for this non-uniform container-soil-water distribution is summarized as follows: Total soil water potential (.psi.) can be defined as the mechanical work required to transfer a unit quantity of water from a standard reference state to the soil location being described. .psi. is the sum of pressure (.psi..sub.p), gravitational (.psi..sub.g), and osmotic (.psi..sub.o) potentials. If soil water is free to move, it always moves from higher to lower .psi..
Container soils are shallow, often homogeneous, and open to the atmosphere at the top and often the bottom. After a container soil has been irrigated and allowed to drain, the water it retains is at static equilibrium (no water movement) and .psi.is constant throughout. Since the salts (cations and anions) concentration is uniform throughout, .psi..sub.o is also constant. Since drainage is into the atmosphere, .psi..sub.p =0 at the drainage level and a perched water table occurs here (soil is saturated). If .psi..sub.g is set to equal 0 at this (drainage) level, then EQU .psi.=.psi..sub.p +.psi..sub.g +.psi..sub.o